The Utility of spin-labeling methods and high resolution 95 GHz (W-band) EPR is demonstrated as applied to equilibrium denaturing of spin-labeled recombinant human growth hormone. Experiments were carried out with 70 nanoliter vlume 33 [unreadable]M aqueous spin-labeled protein samples in a high-quality-factor (Qv4000) cylindrical resonator. Concentrational sensitivity was comparable to that of X-band (9.5 GHz) EPR. The amount of spin-labeled protein in W-band experiments was only v2 picoMoles. At W-band the field separation observed for spin label spectra corresponding to native and denatured protein states was [unreadable]0.6G which, in principle, allows resolution of the states with interconversion rate as fast as v5 [unreadable]sec. Denaturing curves were derived from isotropic g-factor and line width data and analyzed under a two state model.